1. Field of the Invention
The present invention relates to a DC-DC converter that outputs (creates) a stable voltage using an unstable external power source. DC stands for direct current.
2. Description of the Related Art
A DC-DC converter is known as a device for supplying a stable output voltage of a desired level from an unstable voltage source, irrespective of a variation in an input voltage to the DC-DC converter or a changing output load connected to the DC-DC converter. One example of the unstable voltage source is a rectified line voltage which has an unstable amplitude.
DC-DC converters may generally be categorized into a voltage mode control (VMC) type and a current mode control (CMC) type depending on feedback schemes. In a DC-DC converter of the voltage mode control type, a part of an output voltage is fed back as a control signal to control the duty ratio of a switching element so as to stabilize the output voltage. In a DC-DC converter of the current mode control type, a variation in the current flowing through an inductor is also used for the control of the output voltage in addition to the duty ratio of the switching element in the DC-DC converter. Thus, the DC-DC converter of the current mode control type has many advantages such as excellent line regulation, easy phase compensation, easy current control, and appropriateness to large-capacity implementation owing to parallel connection. As a result, recently, the DC-DC converter of the current mode control type is more used than the DC-DC converter of the voltage mode control type.
Japanese Patent Application Kokai (Laid-open) No. 2007-209103 discloses a DC-DC converter of the current mode control type, which is capable of changing the value of an output voltage over a wide range depending on an input voltage. In the DC-DC converter disclosed in Japanese Patent Application Kokai No. 2007-209103, a voltage drop across an ON resistance of a P-type metal oxide semiconductor (MOS) transistor connected in parallel to a switching element is amplified by a magnification determined by the ratio of two resistors, in order to sense the current flowing through an inductor and in turn to control the output voltage.